1. Field of the Invention
The present invention relates to an acousto-optic modulator, and more particularly, to an acousto-optic modulator equipped with a transducer having a structure in which impedance matching is easy.
2. Description of the Related Art
With a recent increase in the demand for multimedia, various large screen display devices appeared in order to overcome the defects of an existing cathode ray tube (CRT) and cope with the multimedia. However, existing image display means such as the CRT or a liquid crystal display (LCD) are difficult to manufacture, and their resolution is degraded, as they become larger. Thus, there is a limit in the use of the CRT and LCD. A projector is an image display device capable of projecting a picture onto a large screen using the CRT or LCD. However, the projector also has many problems and technical limitations.
Another image display device for a large screen is a laser projector for directly projecting laser light having picture information onto a screen. The laser projector can realize a large screen, has both high contrast and optical efficiency, has no distortion or color errors, has both a luminance and contrast independent of distance, and is applied to large screen HDTV (high definition television). Generally, the laser projector uses a xenon (Xe) lamp, helium-neon (He--Ne) laser, and argon (Ar) laser as an optical source. However, krypton-argon (Kr--Ar) laser has received much attention for simplification of the system.
In general, a color display system is comprised of an optical generation unit, an optical modulation unit, an image signal generation unit, a scanning unit, and a screen unit. A predetermined beam is generated by the optical generation unit, and is incident upon the optical modulation unit. The optical modulation unit modulates the incident beam according to an image signal provided from the image signal generation unit. Here, the modulated beam has pixel-unit information. The scanning unit sequentially and continuously scans the screen unit with the modulated beam, such that an image is displayed on the screen unit.
An acousto-optic modulator (AOM), an electrooptic modulator (EOM), or an acousto-optic tunable filter (AOTF) is included as an optical modulator in the optical modulation units currently in use to make laser beam spots scan the screen unit. Among them, the AOM is the most frequently used at present, and has a simple driving circuit. A typical optical modulation system uses three AOMs. In a laser projection display system, a high-output laser beam is used as an optical source to form a high definition image on a large screen, and an AOM used in this laser projection display system must have a structure which can sufficiently resist high output.
FIG. 1 is a perspective view showing the structure of a conventional AOM.
Referring to FIG. 1, in the conventional AOM, a transducer 12 for generating acousto-elastic waves by an electrode 14 is provided on one side of an ultrasonic medium 10. The transducer 12 is coupled to the ultrasonic medium 10 by an adhesive layer 13 including a silver (Ag) layer, and the electrode 14 made of gold (Au) is installed on the upper surface of the transducer 12. Accordingly, a structure is formed in which the transducer 12 is interposed between the electrode 14 and the adhesive layer 13, and this structure acts as a capacitor.
An acousto-elastic wave absorbing element (not shown) for preventing reflection of ultrasonic waves is installed on the side of the medium opposite to the transducer 12. The ultrasonic medium 10 is made of a material selected from the group consisting of fused quartz, PbMoO.sub.4, TeO.sub.2, Te glass, and Schwer-Flint glass (SF.sub.4). A crystal material, PbMoO.sub.4 or TeO.sub.2, is suitable for relatively high frequencies, and a glass material is used mainly for low frequencies since it is cheap, but has a large propagation loss at a high frequency. A light incident/ emitting surface 10A or 10B passing laser light is optically polished An antireflection layer is generally deposited on the light incident/emitting surface 10A or 10B since on account of a high refractive index of the ultrasonic medium 10 there is a large reflection loss when laser light enters or is emitted.
FIG. 2 is a top view of the electrode 14 installed on the transducer 12 of the conventional AOM.
Referring to FIG. 2, a single electrode is used as the electrode 14 in the conventional AOM. When the length (l) of the electrode is 10 mm and the mean height ((h1+h2)/2) thereof is 0.55 mm, the area (A) of the electrode is 5.5 mm.sup.2.
Meanwhile, impedance matching must be performed to use the AOM together with a driving circuit. The impedance matching has the following objectives: (1) to compensate for frequency mismatching of transducer due to the thickness error so as to resonate with the frequency of a driving circuit; and (2) to effectively transmit RF power by performing impedance matching between the driving circuit and the transducer.
When an impedance Rs of 50 .OMEGA. is required by a 150 MHz AOM, the desired impedance cannot be obtained with one transducer employed as in the conventional AOM.
To be more specific, the relationship between the impedance (Rs and a capacitance (C.sub.0) in the structure of the AOM can be generally expressed by the following Equation 1: ##EQU1##
wherein .omega..sub.0 indicates angular frequency and is equal to 2.pi.f.sub.0 (where, f.sub.0 is 1.1 .lambda. s; fs indicates a center frequency and is equal to 150 MHz in the case of the 150 MHz AOM.
Also, the capacitance (C.sub.0) and the area (A) of the electrode have a relationship expressed by the following Equation 2: ##EQU2##
wherein .di-elect cons..sub.0 indicates a dielectric constant in a vacuum and is equal to 8.8542 .lambda.0.sup.-12 F/m, .di-elect cons..sub..gamma. indicates relative permittivity and is equal to 38.6 in the case of a transducer made of an LiNbO.sub.3 (hereinafter, abbreviated to LN) single crystal of 36.degree. Y cut, and l indicates the distance between electrodes, i.e., the thickness of the dielectric, and is expressed by V/2f.sub.0. Here, V indicates an acoustic speed in the LN single crystal and is equal to 7300 m/s in case of the transducer made of an LN single crystal of 36.degree. Y cut.
The area of an electrode to render an impedance of 50.OMEGA. can be obtained with respect to the given length of an electrode by Equations 1 and 2. Alternatively, the impedance can also be calculated from the area of the electrode.
The impedance in the conventional AOM having such an electrode structure as shown in FIG. 2 is calculated to be 11.30.OMEGA. by applying the above-described relationship.
As described above, there is a difference between the desired impedence of 50.OMEGA. and an impedance value obtained when one transducer is employed as in the conventional AOM. The difference therebetween serves as a significant disadvantage in impedance matching between an AOM and a driving circuit, so that RF power cannot be effectively transmitted.